The present invention relates to the recovery of oil from a subterranean oil-bearing reservoir utilizing steam stimulation, and more particularly, to an improved steam stimulation method for increasing oil recovery by utilizing a foam to reduce the loss of steam and steam supplied heat to permeable zones.
In oil well production, it is a conventional stimulation technique to inject steam into oil wells to increase the production of highly viscous oil from those wells, particularly after it has become uneconomical to produce only free-flowing oil. One common method is the cyclic steam stimulation method, wherein production of oil from a well is periodically interrupted and steam is injected into the well. The steam supplies heat to reduce the viscosity of the oil remaining in the oil-bearing strata surrounding the well so that it will flow more readily into the well for production therefrom. A second method is the direct drive injection of steam into an injection well whereby oil in the oil-bearing strata has its viscosity reduced, and is driven ahead of the steam being injected and produced from a nearby production well.
One of the problems faced with either method of steam injection arises from the varying permeabilities of the reservoir. Where there is a permeable zone with a considerable increase in permeability when compared to the oil-bearing strata, the injected steam will flow into the permeable zone preferentially, or on occasion almost exclusively. Since the oil to be produced may be very largely in the less permeable oil-bearing strata, a considerable quantity of steam will be injected into the well with little success and at great cost in time, money and energy.
Another problem encountered is the loss of a portion of the heat already transferred to the oil-bearing strata by the steam as a result of its conduction away into the permeable zone. The permeable zone may be comprised of a portion of the oil-bearing strata which has been substantially depleted of oil during prior production, and various combinations of highly permeable strata, cavities, voids, channels and fingers in the reservoir.
One attempted solution to at least part of this problem is to plug or seal off the highly permeable zone from the well so that the steam injected into the well is directed only into the less permeable oil-bearing strata. One such method is described in U.S. Pat. No. 3,412,793, which uses a foam formed in situ with a condensible gas, such as steam, as its gas phase. A steam-surfactant mixture is injected into the well and foams in the reservoir by the action of the steam passing through the permeable zone.
While use of such a self-collapsing steam-foam avoids the problem mentioned in the patent of plugging the permeable zone using a water solution of a surface active agent which takes a significantly longer and uncertain period to collapse, a significant disadvantage of the method described in the patent is that it only plugs the portion of the permeable zone adjacent to the well to prevent entry of steam directly from the well. The method does not, however, prevent steam injected into the oil-bearing strata from migrating out of the oil-bearing strata and into the permeable zone at points far removed from the vicinity of the well, which results in a substantial loss of heat content.
In many reservoirs where the portion of the oil-bearing strata to be stimulated extends a substantial distance from the well, the mere injection of additional steam-surfactant mixture into the permeable zone in an attempt to significantly extend the plugged portion is of no avail since as the steam-foam begins forming adjacent to the well it also begins inhibiting the steam from penetrating the permeable zone. As the source of steam to the permeable zone becomes more blocked, less foam is formed and consequently the penetration of foam into the permeable zone is also inhibited. Additionally, because the continued presence of heat is required to prevent the collapse of the foam, the foam which does reach farthest into the permeable zone cools off and collapses early because of condensation and the inability to supply sufficient steam vapor to heat and maintain the foam. Since the portions of the permeable zone most removed from the well are the portions least heated by the steam-foam, the foam therein is the least stable. As will be discussed below, preheating of the permeable zone with a hot gas may help the situation; however, there are certain reservoirs which have void spaces and extremely permeable areas where enough steam-foam or heated gas cannot be injected within practical limits to create pressure and temperature conditions sufficient to maintain the steam-foam for any useful period of time.
Another disadvantage of the method described in the U.S. Pat. No. 3,412,793 is that the full benefit of the heat already transferred to the oil-bearing strata by the steam may be lost by conduction of that heat or a significant portion thereof into the permeable zone. The problem of loss of heat by conduction in a well being stimulated by steam injection is recognized in U.S. Pat. No. 3,412,794. To the extent that the foam being used to plug the permeable zone does extend some distance out from the well into the permeable zone, since steam is commonly used as a gas phase, the foam actually increases the thermally conductivity of the permeable zone and aids in the conduction of heat away from the oil-bearing strata.
Other problems encountered with use of the method described in the U.S. Pat. No. 3,412,793 are discussed in U.S. Pat. No. 4,085,800 and include the need to heat the permeable zone prior to its plugging with foam to avoid using a portion of the steam-surfactant mixture injected just to heat the zone to a high enough temperature to form the foam. Without such preheating, a portion of the steam-surfactant mixture is wasted during the initial stage of operation, and a large amount of liquid is formed inside the permeable zone which will interfere with any subsequent oil production therefrom and will accelerate the heat transfer from the foam, thereby reducing its useful life. The same problems will be encountered with condensible foams other than steam-foam. A solution is suggested in the U.S. Pat. No. 4,085,800 patent of using a hot gas, such as carbon dioxide, nitrogen, methane or other noncondensible gases, to preheat the permeable zone, however, such is costly in terms of time, money and energy expendited.
It will therefore be appreciated that there has long been a need for a steam stimulation method which uses collapsible foam of prolonged stability to prevent the injected steam from entering permeable zones directly from the well and indirectly by migration through the oil-bearing strata, and which further insulates the oil-bearing strata from the permeable zone to prevent loss of heat by conduction. Ideally, the method should not require costly preheating of the permeable zone to avoid the waste of surfactant and the injection of excessive amounts of liquid into the permeable zone. The present invention fulfills these needs, and further provides other related advantages.